Conventional reactive polyurethane hot melt adhesive are utilized in packaging, paper converting, paper construction, bookbinding, cartons, case sealing, construction, automotive, and the like. The reactive polyurethane hot melt adhesives create a very durable product and are particularly useful in the manufacture of packages and textbooks.
Conventional reactive polyurethane hot melt adhesives are moisture-curing or moisture-crosslinking adhesives that are solids at room temperature, but they are applied in the form of a melt. The polymeric constituents of the adhesives contain urethane groups and reactive isocyanate groups. Cooling of the melt results first in rapid physical setting of the hot melt adhesive followed by the chemical reaction of the isocyanate groups still present with moisture from the environment to form a crosslinked infusible adhesive.
Conventional reactive polyurethane hot melt adhesives have certain disadvantages inherent in the system. One of the most serious of these is the residual monomer content of polyisocyanates, more particularly the more volatile diisocyanates. Hot melt adhesives are typically applied at elevated temperatures, for example, at 100° to 170° C. After their application, they rapidly build up a high adhesive strength by cooling and acquire their final properties, in particular their heat distortion resistance and resistance to environmental influences, by the post crosslinking of the polyurethane polymer as a result of reaction of the isocyanate groups with moisture. The conventional reactive hot melt adhesive generally contain significant amounts of unreacted monomeric diisocyanates which are partly expelled in gaseous form at the application temperatures of 85° C. to 200° C., typically 120° C. to 160° C., which are usual in the case of hot melt adhesives. At such temperatures ranges, monomeric diisocyanates have a considerable vapor pressure. The isocyanate vapors formed are toxic as well as irritating and sensitizing effect, so that precautionary measures have to be taken to prevent damage to the health of personnel involved in the application process. These measures, including for example the obligation to ensure that the maximum allowable concentration (MAC) is not exceeded, extraction of the vapors at the point of formation and elimination, are highly cost-intensive. The MAC Commission has even classified various monomeric isocyanates in Category 3 “Substances with Well-Founded Suspicion of Carcinogenic Potential” of Section III (Carcinogenic Workplace Substances) of the MAC List. The Trade Association of the German Chemical Industry published a special code of practice entitled “Polyurethane Production and Processing/Isocyanates” with information on the safe handling of isocyanates. According to this publication, the handling of monomeric diisocyanates and compositions that contain monomeric diisocyanates requires special care, and according to the provisions of the Hazardous Substances Act, monomeric diisocyanates and compositions that contain diisocyanates must be labeled accordingly. However, when the content of monomeric diisocyanates in the composition is very low (less than 0.1 wt. %), there may no longer be an obligation to provide hazardous substance labeling.
Accordingly, the development of reactive polyurethane compositions with a drastically reduced content of monomeric diisocyanates or no monomeric diisocyante content is highly desirable for packaging bookbinding adhesives.